The magnetic trigger ring goes on first and the plastic spacer ring goes on top of those distributors which use a short shank rotor. It takes up any space between the top of the trigger ring and the bottom of the short shank rotor when it is seated in the alignment notch. If you happen to have the longer shank rotor, you won't need to use the spacer as the bottom of the rotor rests on the top of the magnetic trigger ring.
Dave
